Doherty amplifiers are amplifiers commonly used in wireless communication systems. Today, for example, Doherty amplifiers are used increasingly in base stations that enable the operation of wireless communications networks. Doherty amplifiers are suitable for use in such applications because the amplifiers include separate amplification paths—typically a carrier path and a peaking path. The two paths are configured to operate at different classes. More particularly, the carrier amplification path typically operates in a class AB mode and the peaking amplification path is biased such that it operates in a class C mode. This enables improved power-added efficiency and linearity of the amplifier, as compared to a balanced amplifier, at the power levels commonly encountered in wireless communications applications.
During operation, at low input power levels, the carrier amplification path is conductive, while the peaking path is non-conductive. As the magnitude of an input signal increases, however, eventually the peaking path will become conductive. At this time, the operation of the peaking amplification path affects the load observed by the amplifier's carrier path. This change in apparent load reduces the efficiency of the amplifier's carrier path as well as the overall efficiency of the amplifier.